Custom function fails because of "maximum number of requests per minute"

Custom function fails because of "maximum number of requests per minute"

Hi,

I have a simple workflow - when a new SO is created, a simple custom function copies a value from the customer's custom field (let's call it DEP) to the SO's custom field. Recently, a person doing purchasing noticed that some SOs don't have DEP even though he expects them to have it. So I looked in the logs and noticed that the custom function, although its result was Succeed, has this result:
  1. {
        "code": 43,
    "message": "For security reasons you have been blocked for some time as you have exceeded the maximum number of requests per minute."
}
My emails with support.uk at zohobooks com went nowhere as they keep going back to API, but this is within Books' automation.

BTW the custom function only makes two calls - one is zoho.books.getRecordsByID to get customer's details, and then zoho.books.updateRecord to update the SO. I can't imagine it overflows our limits - we're small business with maybe 5 active users.

I wrote a condition that the end of the CF to email me when it fails and it is rather often. It makes me wonder what other CFs are failing as they're not reported a Failure in Automation > Worflow Logs.